1. Field Of The Invention
This invention relates to the removal of polymeric contaminants from aqueous absorbent processing solutions to reduce or prevent fouling of process apparatus and/or loss of absorbent in said processing solutions. In particular, the invention relates to the treatment of aqueous absorbent solutions used to contact organic gas-containing gas streams and has as a principal purpose the removal of contaminants and fouling materials. This invention more specifically relates to novel processes for removing acid gas from hydrocarbon-containing gases containing said acid gas and one or more polymerizable components and is applicable to the removal of acid gases from downstream effluents derived from the cracking of crude oil or its fractions (C.sub.5 and higher); the cracking of normally gaseous hydrocarbons (C.sub.1 to C.sub.4) including liquefied petroleum gas; the cracking of the individual constituents of crude oil, its fractions and/or the normally gaseous hydrocarbons mentioned above or mixtures of said constituents; and gaseous effluents from coal. The invention more particularly relates to the removal of polymeric materials which form from said polymerizable component in systems utilized for removing acid gas and which tend to foul the process apparatus used, especially reboilers, heat exchangers and other similar apparatus.
2. Description Of The Prior Art
The problems of polymer formation in systems used to remove acid gas from feed gases are well known in the prior art.
As an illustration, two compounds that must be removed in the separation train of an olefins unit are the acid gases H.sub.2 S and CO.sub.2. In a conventional plant, the cracked gas can contain 800 to 1200 ppm CO.sub.2 resulting from the high temperature interaction of hydrocarbons with dilution steam and 20 ppm or less H.sub.2 S. The H.sub.2 S results from the injection of sulfur compounds to suppress the catalysis of water-gas reactions by furnace tube walls or decomposition of sulfur compounds in the feedstock. Acid gases are usually removed from the cracked gas in one of several ways. The acid-gas removal unit can be placed at the front of the separation train or at the end if the columns are operated to send the acid gases with ethylene. In the former case, the acid gases can be removed by contacting the cracked gas with dilute caustic. In the latter case, the acid gases can be separated from ethylene by absorption in an alkanolamine or other regenerative aqueous absorbent in a column at the end of the train. If mercaptans are present in the cracked gas, acid gas removal must be carried out at the front of the separation train or sulfur removal will be required in the C.sub.2 through C.sub.5 product streams which is more difficult, more costly and less effective. Alkanolamine or other regenerative aqueous absorbent units are not normally placed at the front of the separation train because of the formation of sulfur-containing diolefin polymers that rapidly foul heat exchange surfaces which are normally used in such units to conserve energy and regenerate the absorbent.
The reactive compounds (styrene, isoprene, butadiene, cyclopentadiene, etc.) present in a cracked gas stream dictate that any acid-gas removal system operating in front of the separation train will have a polymer formation problem to contend with. Polymer is produced even in caustic systems used for removal of acid gas. However, continual purging and lack of the need for heat exchange surfaces allow caustic systems to operate successfully. Caustic systems of the type used hereinbefore are not regenerative and thus are costly to operate. Caustic systems heretofore used also generate large quantities of waste caustic solution which presents a significant disposal problem. A regenerative aqueous absorbent acid-gas system will operate successfully only if polymer is removed from the system as it forms. Fouling that results from the introduction of polymer into heat exchangers can be prevented by removing polymer as it is formed.
Many diverse attempts have been made to overcome the problems of polymer formation as described above. Illustratively, U.S. Pat. Nos. 3,598,881; 3,696,162; 3,911,082 and 3,926,591 are directed to correcting the problem of polymer formation by the use of hydrocarbon solvents to selectively extract from the feed gas the polymerizable component that causes the polymer problem.
U.S. Pat. No. 1,956,113 is not concerned with polymer formation and describes a technique for reducing thiosulfate salts with hydrogen sulfide in gas absorption systems to precipitate sulfur followed by the removal of the precipitated sulfur by filtration. This patent is not at all concerned with a polymer fouling problem or the treatment of an absorbent solution loaded with acid gas.
Similarly, U.S. Pat. No. 3,139,324 also does not concern itself with a polymer fouling problem and is directed to a technique for preventing frothing when water is used under high pressure to remove acid gases from gas mixtures. In this patent a small amount of an alkanolamine having no nitrogen-bonded hydrogen and a small amount of a polybasic mineral acid are added to the water.
U.S. Pat. No. 1,944,122 describes a method for controlling foaming in a gas scrubbing operation involving the use of activated carbon and vegetable oil and does not address itself to a polymer fouling problem.
U.S. Pat. No. 3,535,263 teaches the addition of particular types of polyhydroxycarboxylic acids to aqueous ethanolamine solutions to forestall degradation of the ethanolamine but is not concerned with a polymer fouling problem.
U.S. Pat. No. 3,808,140 has the object of avoiding corrosion by adding combinations of antimony and vanadium compounds to aqueous alkanolamine solutions but does not refer to a polymer fouling problem.
None of the above-mentioned patents disclose, teach or suggest the method of avoiding a polymer fouling problem by adjusting the pH of a regenerable aqueous absorbent solution loaded with an acid gas to not more than about 8.5 using an acid or acid-forming material and then filtering the resulting solution to remove polymer in the solution.